The present invention relates broadly to conference circuits for use in a telecommunications system, and in particular to a digital voice conferencing apparatus in a time division multiplex system.
In a telecommunications system, the operation of connecting more than two parties together in a single call is known as conferencing, and a circuit for performing such an operation within an exchange or other switching center is referred to herein as a conference circuit. The function of such a conference circuit therefore is to feed to each conferring party the sum of the speech signals from all the other conferencing parties, but not his own speech signal since this would give rise to undesirable side tone effects.
The analog conference bridge is used for adding the audio signals on the multiple telephone circuits. The conference bridge may be implemented by any of several well-known techniques for bringing together and summing audio signals. The conference bridge is used to maintain the audio signal level for multiple circuits as opposed to direct summing at each telephone as in an extension telephone connection. The extension telephone principle alters the impedance level as each telephone is connected and thus impairs voice signal strength.
In the conventional method of conferencing analog telephone systems, each user sends electrical signals either digital or analog to the central conference bridge. The conference bridge sums the inputs from each conference and returns a signal to each conferee. The signal returned to each conferee includes all conference users minus that particular users input. For example, in a three party conference, user A hears the sum of B+C, not A+B+C. This is because a telephone talker hears himself via a side tone. If he also heard his signal as part of the conference, an objectionable echo could result. In a conventional conference, a conference bridge is required for each conference and the bridge must have the capacity to support all the users who would like to participate in the conference. Furthermore, only the conference bridge had access to all individual signals in the conference and each user's line connecting him to the conference could only support a single signal.
In a system wherein the conventional conferencing method is adapted to a TDM architecture, the actual implementation of this type of conferencing, takes no advantage of the fact that in TDM systems all users have access to all the signal packets. Each user sends data to a central conference bridge which sums the input from each conferee and returns a signal to each conferee. If the data rate of the digitized voice is R, then the bandwidth necessary to support an n party conference is 2nR.